Intermittent or continuous methods of inhibiting the corrosion of steel contained in concrete structures are described. The equipment necessary to effect these methods can be incorporated into the structure during construction or retro-fitted to existing structures. Cathodic protection systems are routinely used in the art, and it is known that impregnated corrosion inhibitors are effective to slow the damage due to corrosion by exposure to the atmosphere, but the unexpectedly beneficial effect of combining the two technologies was not known.
This application is directed to a system for combining delivery of corrosion inhibitors with the cathodic protection of reinforcing concrete members referred to as xe2x80x9crebarsxe2x80x9d in conventionally reinforced concrete structures. Such rebars are produced from mild steel (also referred to as xe2x80x9cblack steelxe2x80x9d) which has less than 1% carbon and less than 2% of alloying elements, combined. More particularly the invention teaches several methods of providing desirable corrosion protection with cathodic protection which may be immediately commenced on newly embedded rebars in reinforced and/or prestressed concrete structures, that is, structures such as bridges, buildings including power stations, marine structures such as docks, and roadways which are yet to be built; or, the system may be used on aging reinforced concrete structures contaminated with salts formed by reaction of the concrete with atmospheric pollutants.
A system is provided for controlling corrosion of steel-reinforced concrete which is contaminated by sulfur oxides, nitrogen oxides, hydrogen sulfide, chlorides and carbonates, and road treatment salts such as sodium chloride and potassium chloride, all of which permeate the concrete structure and attack the steel rebars. This invention combines impregnating the surface of a concrete structure with an inhibitor using an electrical driving force, and thereafter cathodically protecting the structure either with a sacrificial anode, or, with an impressed current. For even better protection, a heavily contaminated structure is cleansed with an electroosmotic treatment which removes detrimental anions in the concrete. With the corrosivity of the environment surrounding the steel greatly diminished due to the electroosmotic treatment, subsequent impregnation with a corrosion inhibitor and using an impressed cathodic current as needed, is found to be more economical than using any of the processes separately.
The inhibitor used may be any one of the compounds known to be effective to inhibit the corrosion of steel in concrete. Such compounds are disclosed in xe2x80x9cCement,xe2x80x9d Encyclopedia of Chemical Technology (Kirk-Othmer; eds, John Wiley and Sons, Inc., NY, N.Y., 5th ed., 1993) vol. 5, pp. 564-598; ACI Manual of Concrete Practice, Part 1xe2x80x941995 (American Concrete Institute, Detroit, Mich. 48219); Encyclopedia of Polymer Science and Technology, vol. 10, pp. 597-615 (John Wiley and Sons, NY, N.Y. 1969) and other texts. Commonly used are inorganic nitrites such as calcium nitrite which may contain minor amounts of sodium nitrite; calcium formate and sodium nitrite, optionally with triethanolamine or sodium benzoate; inorganic nitrite and an ester of phosphoric acid and/or an ester of boric acid; an oil-in-water emulsion wherein the oil phase comprises an unsaturated fatty acid ester and ethoxylated nonyl phenol and the ester of an aliphatic carboxylic acid with a mono-, di- or trihydric alcohol and the water phase comprises a saturated fatty acid, an amphoteric compound, a glycol and a soap; amidoamines which are oligomeric polyamides having primary amine functionality and which are the reaction product of polyalkylenepolyamines and short-chain alkanedioic acids or reactive derivatives thereof; etc. Most preferably the inhibitor is ionizable in aqueous solution, but organic compounds which are not ionizable may also be used in combination with an electrolyte which will xe2x80x9ccarryxe2x80x9d the inhibitor into the concrete.
To provide a basis for comparing the effect of combining processes in which the conditions are different, efficiency of the processes to combat corrosion is used as a common parameter. xe2x80x9cEfficiencyxe2x80x9d is stated as being zero when there is no protection of any kind; efficiency is defined as the amount of metal which was not lost because of protection, divided by the amount of metal which would be lost with no protection, or:
(corrosion rate with no protection)xe2x88x92(corrosion rate with protection) divided by (corrosion rate with no protection).
The following terms are used in this disclosure:
xe2x80x9cEcxe2x80x9d refers to the corrosion potential of the rebar. Ec is measured with a reference electrode placed in contact with the circumferential surface of the concrete sample. It is written negative relative to a standard hydrogen electrode.
xe2x80x9cEpxe2x80x9d refers to the potential at which an effective impressed current for cathodic protection is to be supplied.
xe2x80x9cCDxe2x80x9d: current density=current divided by the superficial area of the rebar in contact with concrete.
xe2x80x9cCPxe2x80x9d: impressed current for cathodic protection, identified separately when different.
xe2x80x9cEP-1xe2x80x9d and xe2x80x9cEP-2xe2x80x9d: direct current provided in separate circuits for electroosmotic treatment; EP-1 removes contaminant anions from the concrete, EP-2 delivers inhibitor cations to the reinforcing members.
xe2x80x9cELxe2x80x9d refers to electrolyte in which samples are immersedxe2x80x94the specific electrolyte, and the sequence in which it is used is specified in each example. EL-1 refers to an aggressive saline solution; EL-2 refers to a solution of a known corrosion inhibitor.
It has been discovered that a steel-reinforced structure is protected against deterioration when a first cathodic impressed current (CP-1) is applied between a primary anode disposed adjacent an outer surface of the reinforced concrete, and, the steel of the structure, at a potential in the range from 50 mV to about 350 mV numerically greater than the corrosion potential Ec measured; the steel functions as a primary cathode; the structure is substantially saturated with a solution of a corrosion inhibitor; preferably the structure is continuously bathed in the inhibitor solution; flow of the first impressed current is maintained until flow is relatively constant at a level at least one-half the level at which the first impressed current was initiated. A reference electrode is used to indicate the corrosion potential at the rebars. The concentration of ions is sensed by measurement of the current flow while maintaining a chosen voltage.
Excellent protection against deterioration of the concrete structure is also provided with a secondary cathode and a secondary anode, both adjacent but exteriorly disposed relative to the structure, allowing a direct first electroosmotic current and an impressed cathodic current to be applied concurrently; the direct first electroosmotic current is applied at a chosen voltage non-injurious to humans, between the secondary electrodes, at a level sufficient to drive cations or anions of the inhibitor into the concrete; when flow of the first electroosmotic current decreases at least by one-half, the direct impressed cathodic current is applied. If desired, the first electroosmotic current may then be switched off (when it decreases at least by one-half) and then the direct impressed cathodic current is applied.
For badly contaminated structures, prior to applying the direct first electroosmotic current, a direct second electroosmotic current between the secondary electrodes is applied at a chosen third voltage non-injurious to humans, at a level sufficient to remove contaminant anions in the concrete; the second electroosmotic current is maintained at essentially constant voltage until its flow decreases by least by one-half.
It is therefore a general object of this invention to provide a cathodic protection system which may be used in combination with an impregnation system for impregnating a corrosion inhibitor, either successively, or essentially concurrently; for even better corrosion protection, the foregoing systems may be preceded by electroosmotic treatment, or, if the economics justify doing so, may be used essentially concurrently with a set of secondary electrodes.
When an impressed current is used, a determination that the current density is too high to be economical, results in a control system making the electrical connection between the secondary electrodes. When the sensing means senses that the concentration of inhibitor corresponding to a measured current density is sufficiently low, the supplemental anode is disconnected. If a sacrificial anode is used for cathodic protection, the galvanic circuit with the rebars is reestablished. If desired, the galvanic circuit with the rebars and anode, whether sacrificial or inert, may be maintained while the concrete is being impregnated with inhibitor.
If the concrete structure is heavily contaminated, electroosmotic treatment is commenced before impregnation with inhibitor. The circuit for electroosmosis is turned off when the concentration of salts is sensed to have dropped to a low enough level that an impressed cathodic current may be turned on and maintained at a certain level, typically in the range from about 150 mV to less than 300 mV lower than the corrosion potential of the rebars until the current density rises to more than 100 mA/m2. The impressed current may then be turned off. Control of the system is effected with a programmable control means associated with the power source.